A radiant tube is a device that is used to heat a process chamber in a furnace. The tube extends across the process chamber. A burner is fired into one end of the tube, or a pair of burners alternately fire into opposite ends of the tube. In each case combustion proceeds downstream along the length of the tube from the firing burner toward the other end of the tube. The process chamber is heated by thermal energy that radiates from the tube as a result of combustion within the tube.